Solvent cleaning system

ABSTRACT

A improved solvent cleaning method and system which is truly a closed system in which the object to be cleaned is placed in a chamber and subjected to a negative gauge pressure to remove air and other non-condensible gases after which a solvent is introduced to the evacuated chamber and the object is cleaned and then the solvent is recovered from the object and chamber within the closed system before the clean object is removed.

RELATED INVENTIONS

This is a continuation in part of application Ser. No. 08/053,161 filedApr. 26, 1993 which is a divisional application of Ser. No. 07/787,935filed Nov. 5, 1991 issued as U.S. Pat. No. 5,240,507 on Aug. 31, 1993.

FIELD OF INVENTION

This invention relates to an improved cleaning system, and moreparticularly to a closed solvent cleaning method and system whichvirtually eliminates the mixture of the solvent with air throughout thecleaning operation. Eliminating air from the cleaning process and thesolvent recovery and solvent cleaning process allows complete recoveryof the vapors by conventional condensing thereby controlling emissionsto the surroundings.

BACKGROUND OF INVENTION

Cleaning operations are becoming more and more of a burden on industrybecause of the ever-stricter environmental requirements for dispositionof compounds used in the cleaning operations and resulting effluents.Cleaning operations effected include those involving clothing, rugs andfurnishings, as well as those of a more industrial nature such asinvolving the cleaning and degreasing of metals, ceramics, plastics andother materials. Solvent cleaning processes, those using a solvent todegrease and clean, are the most prevalent. There are two types ofsolvent cleaning processes: open and closed. Open systems are still themost commonly used, but their appeal is shrinking with increasingdemands of environmental safety. Open systems include such approaches assolvent vapor degreasing, solvent ultrasonic cleaning, cold or hotsolvent dipped and solvent spray systems. These systems suffer from anumber of shortcomings, among the most important of which are thecontamination of the environment and the cost of constantly replenishingthe non-recoverable solvent. In addition, the cost of equipment tocontain the vapor and to properly dispose of the vapor and liquid wasteis becoming more and more formidable.

The closed systems, so-called, attempt to combat these problems but withindifferent success. The loss of solvent as vapor and liquid stilloccurs, even in a so-called closed system because the vapor escapes whenthe cleaned parts are removed and the parts carry off solvent whichclings to their surfaces and resides in the pores of the material.Further, attempts to recover the solvent are expensive and less thantotally successful. For, even though the system is "closed" when thesolvent is introduced to the closed chamber, it mixes with the airthere. After the cleaning operation, the solvent in liquid form can beeasily separated from the air, but not so with solvent in vapor form.That requires a major effort. Even if the air and solvent are condensed,only a small amount of solvent can be recovered.

Therefore, these systems are not truly closed. Incineration is onetechnique for getting rid of solvent but that requires significantinvestment in special equipment, it uses extra heat energy, and solventis lost and must then be replenished. Steam stripping is a techniquewhich actually recovers the solvent, but it too requires specialequipment and heat energy to make the steam. In addition, the steam mustbe condensed to water and then separated from the solvent.

Thus, conventional cleaning systems have problems in the area ofhazardous emissions and solvent recovery. They are generally limited tooperating at specific temperatures and pressures. They typically do notdry all the solvent off the objects before exposing them to theatmosphere. They utilize heat energy during a substantial part, if notall, of the cleaning cycle. Conventional systems also need a great dealof solvent to fill their cleaning tanks and require additional energyinput to pump the solvent through the system. In addition, standardsolvent vapor cleaning systems must use solvents whose vapors areheavier than air. These vapors are confined in a blanket over theboiling solvent by using expensive refrigerator coils and by limitingthe dimensions of the system tank. These systems operate at fixedtemperatures which are determined by the boiling point of the solvent atatmospheric pressure.

A different approach, known as vacuum degreasing, avoids some of theproblems of solvent cleaning. In this approach the contaminants areexposed to a high temperature, low-pressure environment in order toreach pressures below the vapor pressure of the contaminant. Essentiallythe contaminant is boiled off the parts. The problem here, however, isthat some contaminants generally have a very low vapor pressure.Consequently, extremely high vacuums and/or temperatures are required.Although solvents are not emitted, the contaminant itself is oftenemitted which can become a problem. And, this process is generallycostly due to the sub torr pressures and high temperatures required. Inmany cases, non-volatile residue, either present in the contaminantoriginally (i.e., sulfur residue) or residue resulting from a breakdownof the contaminant due to the high temperature requirements (e.g. carbondeposits) are often left behind on the parts. The pollution abatementenergy costs and cleaning efficiency requirements strongly limit theapplications of such a system.

SUMMARY OF INVENTION

It is therefore an object of this invention to provide an improvedclosed circuit solvent cleaning system and method.

It is a further object of this invention to provide such an improvedclosed circuit solvent cleaning system and method which employs thesolvent in either vapor or liquid form or both.

It is a further object of this invention provide such an improved closedcircuit solvent cleaning system and method which enables solventrecovery and limits hazardous emissions.

It is a further object of this invention provide such an improved closedcircuit solvent cleaning system which is operative at varioustemperatures to accommodate different objects to be cleaned anddifferent solvents.

It is a further object of this invention to provide such an improvedclosed circuit solvent cleaning system which is operative at varioustemperatures and pressures other than the boiling point at atmosphericpressure of the solvent used.

It is a further object of this invention provide such an improved closedcircuit solvent cleaning system and method in which solvent is dried offthe object before any venting to atmosphere occurs.

It is a further object of this invention to provide such an improvedclosed circuit solvent cleaning system and method which more efficientlycleans by condensation of vapor on parts causing a vapor wash andcleaning.

It is a further object of this invention provide such an improved closedcircuit solvent cleaning system and method which is much more efficientbecause heat is required for much less of the time during the cycle.

It is a further object of this invention provide such an improved closedcircuit solvent cleaning system and method which uses much less solvent.

It is a further object of this invention provide such an improved closedcircuit solvent cleaning system and method which removes volatilecontaminants prior to any exposure to solvent.

It is a further object of this invention provide such an improved closedcircuit solvent cleaning system and method which employs internallygenerated differential pressures to drive the solvent through thesystem.

It is a further object of this invention provide such an improved closedcircuit solvent cleaning system and method which is not restricted tooperate with any particular solvent.

It is a further object of this invention provide such an improved closedcircuit solvent cleaning system and method which virtually eliminatesmixture of the solvent with air throughout the cleaning operation andtherefore eliminates the difficult step of separating the solvent fromthe air after the cleaning operation is completed.

It is a further object of this invention to provide such an improvedclosed circuit solvent cleaning system and method which allows parts tobe cleaned by solvent immersion or solvent spray, or both.

It is a further object of this invention to provide such an improvedclosed circuit solvent cleaning system and method which throttlessolvent vapors about the parts to be cleaned thereby aiding in dryingthe parts after cleaning.

It is a further object of this invention to provide such an improvedclosed circuit solvent cleaning system which allows any contaminatedsolvent to also be cleaned, and cleaned within the closed circuit.

The invention results from the realization that an efficient,economical, safe, and environmentally sound solvent cleaning techniquewhich is a truly closed operation can be effected, without the burden ofhaving to separate the solvent from air after the cleaning operation, byevacuating a cleaning chamber containing parts to be cleaned before thesolvent is introduced so that the solvent and air never meet, thuseliminating exposure to atmosphere. The airless solvent can then beeasily extracted and condensed and even cleaned for reuse prior to theexposure of the chamber and parts to air during the removal of partsfrom the chamber after the cleaning operation has been completed.

This invention features and may suitably comprise, consist of, orconsist essentially of a closed circuit solvent cleaning methodcomprising the steps of placing the object to be cleaned in a chamberand subjecting the chamber to a negative gauge pressure to remove airand other non-condensible gases. Following this, a solvent is introducedto the evacuated chamber and the object is cleaned. The solvent is thenrecovered from the object and chamber and the chamber is vented toatmosphere and the clean object is removed.

In a preferred embodiment the negative gauge pressure is in the range ofatmospheric to zero atmospheric absolute. The solvent may be introducedin a vapor state or a liquid state, or both. The temperature of thechamber may be varied to control the temperature and vapor density ofthe solvent to increase or decrease the penetration of the solvent intothe object to be cleaned and to create more or less pressure that can beused to drive the solvent through the closed system. Recovering thesolvent includes withdrawing from the chamber the solvent in liquidstate including the contaminants, and then drawing off from the chamberthe solvent in the vapor state. Withdrawing the liquid solvent mayinclude maintaining the chamber at an elevated temperature to generateincreased pressure in the chamber and positively drive out the solventin the liquid state with the contaminants. The drawing off of thesolvent in the vapor state may include drying the object of solvent.

The invention also features an apparatus for accomplishing the techniqueof the invention including a closed solvent cleaning system. There is achamber for holding an object to be cleaned and means for applying anegative gauge pressure to the chamber to remove air and othernon-condensible gases. There are also means for introducing to thechamber the solvent for cleaning the object and means for recovering thesolvent from the object and chamber. Storage means stores the recoveredsolvent.

In a preferred embodiment, the chamber may include a heat exchanger forvarying the temperature of the chamber. The means for applying anegative gauge pressure may include a vacuum pump and the means forintroducing the solvent may include a valve means associated with thestorage means. The means for recovering may include a drain forextracting the solvent liquid and contaminants and may include means forextracting the solvent vapor as well. The storage means may include onereservoir for receiving the solvent vapor and a second reservoir forreceiving the solvent liquid, and there may be means for condensing thesolvent vapor.

The system may also include means for spraying solvent over the parts tobe cleaned, for immersing the parts in solvent, or both. In addition,there may be means for throttling solvent vapors into the chamber toassist in drying parts. Means for processing and cleaning contaminatedsolvent within the closed circuit includes a distilling tank and aholding tank and means for heating the distillation tank, distillingsolvent vapor and urging the distilled vapor into the holding tank.

DISCLOSURE OF PREFERRED EMBODIMENT

Other objects, features and advantages will occur to those skilled inthe art from the following description of a preferred embodiment and theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of a closed circuit solvent cleaningsystem according to this invention; and

FIG. 2 is a flow chart depicting the operation of the improved closedcircuit solvent cleaning system of FIG. 1 according to this invention.

The invention may be accomplished in a closed circuit solvent cleaningtechnique in which the object to be cleaned is placed in a chamber, andthen subjecting the chamber to a negative gauge pressure in order toremove the air and other non-condensible gases from the chamber. Solventcleaning as used herein includes vapor degreasing, clothing drycleaning,and solvent immersion washing. In conventional solvent cleaningsystems, most of the problems with the loss of expensive solvent resultfrom the fact that the solvent vapors become mixed with air. The airthen is either vented, requiting expensive and complex equipment forpreventing environmental contamination, or the air with the vapor is putthrough a recovery process which can be equally expensive and complex,in order to recover the expensive solvent and cleanse the air before itis vented to atmosphere. This invention results from the realizationthat the entire problem can be eliminated or at least dramaticallyreduced by not allowing the solvent vapors to become mixed with air atall: to prevent any mixing at any time of the solvent with the air. Thisis done by evacuating the chamber of air and other non-condensible gasesbefore any solvent is introduced so that the chamber contains only theobject to be cleaned. The chamber is evacuated by employing a negativepressure which generally is less than the vapor pressure at the presentoperating temperature of the solvent and the contaminants to be removedsuch as water vapor and non-condensible gases. Sub-torr levels are notgenerally required. Torr levels of 0-700 torr are preferred. Ranges of10-500 torr are workable and levels of 100 ton or less have had goodresults. After the solvent has been introduced and the part cleaned, thesolvent is then recovered from the chamber and the object before thechamber is opened and vented to the atmosphere. Thus there is nocontaminated air and the solvent may be recovered easily without thenecessity to separate it from air. Depending on operating temperature,typically a negative gauge pressure in the range of atmospheric toslightly above zero atmospheric absolute is sufficient.

With the chamber evacuated, it is typically at a lower pressure than thesolvent, which is stored in a holding tank. The opening of a valve thenpermits the solvent to flow or to flash in vapor form from the higher tothe lower pressure in the chamber without requiring pumping equipmentand the added expense of the energy required. Liquid as well as vaporphase solvent can be introduced to the chamber at this point. Once thesolvent has been introduced into the chamber, the object is cleaned.Cleaning can be accomplished by vapor or liquid immersion and/or bysolvent spray in the chamber. Because of the control afforded by theclosed operation of this invention, the solvent can be heated or cooledand/or the pressure Can be increased or decreased to whatever levels aredesirable for a particular cleaning task. For example, the pressure inthe chamber can be increased above atmospheric to enhance cleaningefficiency or the temperature could be increased above the ambienttemperature. Then before the object is removed or there is any ventingto atmosphere, the solvent is returned to the storage tank andrecovered. The liquid may be recovered first, as it typically willreside at the bottom of the tank and contain in it the contaminants thathave been removed from the, clean part. This liquid solvent can bedrained by gravity back to the holding tank or the temperature of thechamber can be increased to increase the pressure in the chamber so thatthe liquid solvent can be driven back to the holding tank.

Subsequent to this the vapor may be drawn off and fed back to a separateholding tank or a separate compartment of the original holding tank.Included in this step may be the introduction of solvent vapor,preferably by throttling the vapor though a valve to assist in dryingparts and removing solvent from the parts prior to removing the partsfrom the chamber. Separation is desirable between the vapor recovery andthe liquid recovery because the vapor in this case is effectively adistillation product and will remain quite clean compared to the liquidwhich contains the contaminants. The vapor may be condensed in itstravel back to the holding tank. Following the removal of the solvent inall its phases, the chamber may be opened to atmosphere so that theparts can be removed. The vapor removal from the chamber is sufficientlythorough so that a drying of the parts occurs as well as the removal ofthe vapor from the chamber. A heat exchanger or similar device may beassociated with the chamber in order to control the temperature of thecleaning process. Thus certain materials which cannot withstand elevatedtemperatures or perhaps even room temperatures during the cleaningprocess may be accommodated by simply cooling the chamber. The heatexchanger may also increase the temperature in the chamber to accomplisha number of different goals. Increased temperature increases thepenetration of the vapor into the part to be cleaned and thus enhancesthe cleaning function. Increased temperature also increases the chemicaland physical cleaning characteristics of the solvent and thus enhancesthe cleaning function. Increased temperature also increases the vaporpressure in the chamber which can be used to drive out the liquid andvapor solvents as indicated previously. The temperature or cooling thatis applied to the chamber may be applied only during the cleaning cycleat a great saving of energy over those systems that have heat, forexample, applied during the entire operation.

One of the advantages of such a system is that it can work with solventsin the vapor form, in the liquid form or both, and it can work with avariety of different solvents, e.g., 1.1.1. trichloroethane,trichloroethylene, methylene chloride, perchloroethylene, Freon,aldehydes, alcohols, amines, ketones, aromatics, or other solvents whichmay or may not be heavier than air. Another important feature of thesystem according to this invention is that it overcomes the problem ofsolvent and/or liquid penetrating small areas such as tapped holes inthe parts to be cleaned. Since the air is removed before solvent isintroduced to the part in the system of this.

Such a system enables solvent recovery, which is increasingly importantas solvent prices increase, and also limits hazard emissions, which isequally important as the regulations for emissions become more stringentand the cost of abiding by those regulations becomes more expensive.

Added features of this invention are that it uses much less solventsince the vapor is flashed into the cleaning chamber with at worst onlya small amount of liquid; that is, the entire cleaning chamber or tankdoes not have to be filled as is often the case with open circuitsystems.

Another important advantage of the subject closed circuit system is thatthe initial evacuation of the chamber after the parts are inserted forcleaning but before the solvent has been introduced, operates to removevolatile contaminants that may be, associated with the parts even beforethe solvent is introduced.

The system 10, FIG. 1 for implementing the technique of this inventionincludes a cleaning or degreasing tank or chamber 12, which includes aheat exchanger 14. Valve 16 controls the inlet flow of the heating orcooling fluid. In this specific embodiment it is a heating fluidobtained from steam source 18. Other options for heating chamber 12include electrical heaters or other heat transfer liquids such as oil. Apart to be cleaned 20 may be placed on a suitable support 22 withinchamber 12. Valve 66 operates to vent chamber 52 to the atmosphere. Pump26 is used to apply a negative gauge pressure to chamber 12 when it isoperating as a vacuum pump. An activated charcoal filter may be added toabsorb any residual solvent vapors before they enter the vacuum pump.Valve 30 operates to vent the outflow through vacuum pump 26 to carbonfilter 28 while valve 32, in a different portion of the cycle, directsthe outflow to condenser 34 through vacuum pump 36 to holding tank 38.Holding tank 38 provided with a heater 40 communicates through valves 42and 44, respectively, that communicates through pump 46 with chamber120.

Chiller unit 48 is used to provide coolant to heat exchangers 34, 50,and 52. Other conventional means for providing Cooling include coldwater directly from a source of cold water or from a cooling tower.Valves 54 and 56 are used to purge air from holding tank 38 anddistillation tank 58 respectively, and deliver it to a carbon filter 28or a similar filter before it is vented to atmosphere. A third input tocarbon filter 28 may be delivered through valve 30 which may beinterconnected with degreasing tank or chamber 12 so that the outflowfrom vacuum pump 26 upon the evacuation of chamber 12 can be filteredfirst through carbon filter 28 before it is vented to atmosphere. Thisis especially important if there are volatile toxic contaminantsassociated with the parts that can be drawn off by the initialevacuation of chamber 12.

In operation, with the solvent stored in distilling tank 58, heater 60is activated to increase the temperature of the solvent such astetrachloroethylene to 100° C., producing a 400 torr vapor pressure.Heating is accomplished by steam directed though valve 64 from steamsource 18. Heating can be accomplished by other conventional means suchas electric heaters or heat transfer fluids. Valve 24 is then opened,venting chamber 12 to atmosphere, part 20 is placed on support 22 inchamber 12, valve 24 is closed and vacuum pump 26 is operated. All ofthe air and non-condensible gases and any volatile contaminants aredrawn off by vacuum pump 26 and are directed by open valve 30 directlyto atmosphere or, alternatively, through and carbon filter 28, and thento atmosphere. Vacuum pump 26 is then shut off. Since thetetrachloroethylene solvent in distilling tank 58 is at 100° C., with a400 torr vapor pressure, when valve 62 is opened, the vapor flashes intochamber 12 so that the vapor 66 fills chamber 12 and condenses on andcleans part 20. If desired, liquid solvent 68 may also be introduced byopening valve 70 and partially or fully filing chamber 12 to submergepart 20 for liquid cleaning. If spraying is desired, valves 44 and 42are opened and pump 46 is operated to draw solvent through sprayer 84 inchamber 12 or valve 72 is opened to draw solvent from tank 72. Valves 16and 64 may now be opened and steam source 18 is activated to increasethe temperature of chamber 12 to approximately 121° C, providing a 760torr or 1 atmosphere pressure in the chamber during cleaning. After thecleaning cycle has been completed, the steam source 18 may be shut downand valves 16 and 64 closed. Valve 62 may be periodically opened toallow the liquid solvent 68 to gravity drain back through open valve 62to distilling tank 58. Or more typically, the increased pressure of 760torr will drive the liquid with its contaminants back into thedistilling tank 58. The pressure in chamber 12 now drops to about 400torr.

Following this, vacuum pump 36 may be operated with heat exchanger 34 onand valve 32 open. This draws off the vapors 66 in tank 12 including thevapors associated with object 20 so that it is dried during thisprocess. The vapor, being virtually pure, is condensed in condenser 34and delivered back to holding tank 38, which stores only clean solventwhich may be used when the solvent in distilling tank 58 becomescontaminated and must be removed and processed. Periodic purging of theair in tanks 38 and 56 is accomplished through valves 54 and 56. Dryingof part 20 in chamber 12 may be assisted by throttling vapor solvent intank 58 through valve 62 while simultaneously pulling vapor out ofchamber 12 through valve 78 and condensor 34 by means of pump 36.

Finally, vacuum pump 34 is stopped and valve 24 is opened to ventchamber 12 to atmosphere and part 20 is removed, having been dried andcleaned without introducing any hazardous waste to the atmosphere.Simultaneously, the solvent has been fully recovered with a minimum ofeffort and expense since it was not mixed with air and there is no needto undertake the expensive an complex procedures required to separatesolvent from air and clean the air of the solvent contaminants.

When the solvent in distilling tank 58 becomes contaminated: the solventcan be distilled by opening valve 64 from steam source 18, and flashingvapors through open valves 56 and 78. Pump 36 pulls vapors throughcondensor 34 and sends clean solvent into holding tank 38. Upon solventrecovery, contaminants can be removed from distillation tank 58 throughvalve 82. Clean solvent can then be returned to distilling tank 58through valves 42 and 72 for reuse.

There is shown in FIG. 2 a flow chart depicting the operation of system10, FIG. 1, of this invention. The object to be cleaned, such as a pieceof clothing or a manufactured part, is placed in the cleaning chamber instep 100. Then a negative gauge pressure is applied in step 102. Thisremoves air and other non-condensible gases and it also removes anyvolatile contaminants. The gasses evacuated from the chamber at thispoint can be passed through suitable filters if this is necessary. Thenegative gauge pressure is typically between atmospheric and zeroatmospheric absolute. Pressures in the range of 10 torr appear to besufficient. Following this, the solvent is introduced in step 104. Thiscan be done in vapor or liquid form, or both. Then the object iscleaned, step 106, for an appropriate period of time. During this time,the temperature can be varied to favor the appropriate conditions forthe material or object being cleaned and also to improve vapor densityand penetration of the solvent into the object. The temperature increaseor decrease is effected only during the cleaning operation so that thereis a substantial saving in energy. There can also be a substantialsaving in energy by the fact that an increased temperature of thechamber increases the differential pressure between the two chambers tothe point where that differential pressure alone could be used to driveout the solvent after the cleaning operation is done. Typically, withthe solvent being present partially as a liquid and partially as avapor, the solvent is recovered in step 108 by first removing the liquidwhich contains the contaminants, and then removing the vapor which isvirtually clean since it is a distillation product. A complete removalof the vapor at this point also effects a drying of the object, whichfurther minimizes the contamination of the environment with solvents invapor or liquid form that would ordinarily cling to the object. Finally,in step 34, the chamber is opened to atmosphere and the cleaned objectis removed.

When the solvent becomes contaminated, it is distilled by first heatingthe solvent, step 112 and then extracting the solvent vapor step 114.The solvent vapor is condensed, step 176 and stored in a separate tank.The contaminates are then removed, step 118.

Although specific features of the invention are shown in some drawingsand not others, this is for convenience only as each feature may becombined with any or all of the other features in accordance with theinvention.

Other embodiments will occur to those skilled in the art and are withinthe following claims:

What is claimed is:
 1. A closed circuit solvent cleaning systemcomprising:a chamber for holding an object to be cleaned; means forapplying a negative gauge pressure to the chamber to remove air andother non-condensable gases; means for introducing a solvent to theevacuated chamber; means for cleaning the object while maintaining anairless environment within the chamber; means for recovering the solventfrom the object and chamber; storage means for storing the recoveredsolvent; and means for processing and cleaning contaminated solventwithin the closed circuit.
 2. The system of claim 1 in which said meansfor introducing includes means for supplying solvent vapors into thechamber.
 3. The system of claim 1 in which said means for introducingincludes means for supplying liquid solvent to at least partially fillsaid chamber.
 4. The system of claim 1 in which said means forintroducing includes means for spraying solvent about the object to becleaned.
 5. The system of claim 1 in which said means for introducingsolvent includes means for preheating the solvent before it isintroduced into the chamber.
 6. The system of claim 1 in which saidmeans for cleaning includes means for heating the chamber duringcleaning.
 7. The system of claim 1 in which said storage means includesa distilling tank and a holding tank.
 8. The system of claim 7 in whichsaid means for recovering the solvent from the object and the chamberincludes means for urging liquid solvent in the chamber to flow intosaid distilling tank.
 9. The system of claim 7 in which said means forrecovering the solvent from the object and the chamber includes meansfor drawing vapor from the chamber to said holding tank.
 10. The systemof claim 9 in which said means for drawing vapor from the chamber tosaid holding tank further includes means for condensing the vapor beforedelivery to the holding tank.
 11. The system of claim 1 in which saidmeans for processing and cleaning contaminated solvent includes meansfor heating and distilling the contaminated solvent.
 12. The system ofclaim 11 in which said storage means includes a distillation tank incommunication with a holding tank.
 13. The system of claim 12 in whichsaid means for processing and cleaning contaminated solvent includesmeans for heating said distillation tank, distilling solvent vapor, andurging the distilled vapor into said holding tank.
 14. The system ofclaim 1 further including means for drying the part in the chamberincluding means for throttling vapor solvent into said chamber whilesimultaneously pulling vapor out of chamber within said circuit meansfor processing and cleaning.